Bike Gear Calculator
Compare every chainring and cog combination using gear ratio, gear inches, rollout, or estimated speed at cadence.
Enter the tooth counts that are actually fitted to the bike
Separate multiple chainrings and cassette cogs with commas. Wheel and cadence inputs add gear inches, rollout, and speed without changing the tooth-count ratio.
Overall usable range
Lowest gear
1.06
34 × 32
Highest gear
4.55
50 × 11
Low gear inches
29.2″
Speed range
12–50 km/h
at 90 rpm
Gear ratio matrix
Rows are chainrings; columns are rear cogs. Low and high extremes are highlighted.
Core calculation
How the bike gear calculator works
The front chainring drives the rear cog through the chain. Dividing their tooth counts tells you how many rear-wheel revolutions occur for one crank revolution on a conventional derailleur or single-speed drivetrain.
A bike gear calculator calculates cycling gear ratios, gear inches, development, and speed from chainring size, rear sprocket size, wheel diameter, and cadence. Cyclists use it to compare gearing setups, estimate distance traveled per pedal revolution, and choose gears suited to climbing, flat roads, or high-speed riding.
Gear ratio
ratio = front teeth ÷ rear teeth
A 50 × 25 combination has a ratio of 2.00: the wheel rotates twice per crank revolution.
Gear inches
gear inches = ratio × wheel diameter
A 2.00 ratio on a 27.5-inch wheel equals 55 gear inches.
Gearing-method reference: Bicycle technical writer Sheldon Brown's guide explains the history and formulas behind gear inches, meters of development, and gain ratio, including why crank length is not represented by gear inches alone.
Four useful outputs
Ratio, gear inches, rollout, and speed
Gear ratio
Compares sprocket tooth counts. It is ideal for comparing combinations on the same wheel but ignores tire size.
Gear inches
Adds effective wheel diameter, making similar drivetrains on different wheel sizes easier to compare.
Rollout or development
Shows meters traveled per complete crank revolution: measured wheel circumference × gear ratio.
Speed at cadence
Multiplies rollout by crank revolutions per minute. It is a theoretical no-slip speed, not a promise of road speed.
Choose the right end of the cassette
Low gears for climbing and high gears for speed
| Combination | Effect | Typical use | Tradeoff |
|---|---|---|---|
| Small front × large rear | Low ratio and short rollout | Steep climbs, starts, loose terrain | Lower speed at the same cadence |
| Middle combinations | Moderate ratio | Rolling roads and steady cruising | May overlap with nearby chainring choices |
| Large front × small rear | High ratio and long rollout | Descents, tailwinds, fast flat riding | Requires more force at the pedals |
Avoid extreme cross-chaining when the drivetrain manufacturer advises against it. On multi-chainring bikes, the most diagonal chain combinations may increase noise, wear, or shifting problems even though the mathematical ratio is valid.
Wheel measurement
Why measured tire circumference improves speed estimates
Nominal wheel labels do not equal the exact rolling diameter. Tire width, casing, rim width, pressure, load, and tread can change the distance traveled per revolution. A rollout measurement captures the fitted setup more directly.
1. Mark the tire
Put the valve at the floor and mark the starting point.
2. Roll one revolution
Load the bike normally and roll straight until the valve returns to the floor.
3. Measure in millimeters
Enter the distance as circumference; repeat and average for better precision.
Wheel-size starting points
Approximate bicycle wheel circumference chart
These geometric estimates provide a useful first value when a rollout measurement is unavailable. They use the bead-seat diameter plus twice the labeled tire width; the actual loaded circumference can differ.
| Common label | ETRTO example | Estimated diameter | Estimated circumference | Typical category |
|---|---|---|---|---|
| 700 × 25C | 25-622 | 672 mm | 2,111 mm | Road |
| 700 × 28C | 28-622 | 678 mm | 2,130 mm | Road / all-road |
| 700 × 32C | 32-622 | 686 mm | 2,155 mm | All-road / commuting |
| 700 × 40C | 40-622 | 702 mm | 2,205 mm | Gravel |
| 650B × 47 | 47-584 | 678 mm | 2,130 mm | Gravel / adventure |
| 26 × 2.0 | 50-559 | 659 mm | 2,070 mm | Mountain / utility |
| 27.5 × 2.25 | 57-584 | 698 mm | 2,193 mm | Mountain |
| 29 × 2.25 | 57-622 | 736 mm | 2,312 mm | Mountain |
For a bike computer or a precise speed estimate, measure the fitted tire. Tire labels are nominal, and real circumference changes with rim width, casing, tread, pressure, and rider load.
Device setup reference: Garmin's support guide provides a manufacturer reference table for setting bicycle wheel circumference in millimeters. Use a measured rollout when accuracy matters because published lookup values are still approximations.
Speed lookup
Bike speed by gear inches and cadence
Use this chart when you already know the gear-inch value and want a fast theoretical speed estimate. Values are in kilometers per hour and assume no tire slip.
| Gear inches | 60 rpm | 75 rpm | 90 rpm | 105 rpm |
|---|---|---|---|---|
| 30″ | 8.6 | 10.8 | 12.9 | 15.1 |
| 40″ | 11.5 | 14.4 | 17.2 | 20.1 |
| 50″ | 14.4 | 18.0 | 21.5 | 25.1 |
| 60″ | 17.2 | 21.5 | 25.9 | 30.2 |
| 70″ | 20.1 | 25.1 | 30.2 | 35.2 |
| 80″ | 23.0 | 28.7 | 34.5 | 40.2 |
| 90″ | 25.9 | 32.3 | 38.8 | 45.2 |
| 100″ | 28.7 | 35.9 | 43.1 | 50.3 |
km/h = gear inches × 0.0254 × π × rpm × 60 ÷ 1000
Real speed can be lower because of tire deformation, drivetrain losses, gradient, wind, surface, and the rider's ability to sustain the selected cadence.
Plan a drivetrain change
Choose a rear cog for a target climbing gear
If you know the lowest gear inches you want, work backward from the wheel diameter and chainring. Round the rear-cog result up to reach a gear that is equal to or easier than the target.
Required rear cog
rear teeth = front teeth × wheel diameter ÷ target gear inches
For a 32T chainring, 29-inch wheel, and 20-inch target: 32 × 29 ÷ 20 = 46.4, so choose at least a 47T cog mathematically.
Maximum chainring for a target
front teeth = target gear inches × rear teeth ÷ wheel diameter
Round the chainring result down when the goal is a gear no harder than the target, then check which compatible sizes exist.
| Setup | Target low gear | Calculated rear cog | Minimum whole cog | Resulting gear inches |
|---|---|---|---|---|
| 32T front · 29″ wheel | 18″ | 51.6T | 52T | 17.8″ |
| 32T front · 29″ wheel | 20″ | 46.4T | 47T | 19.7″ |
| 32T front · 29″ wheel | 22″ | 42.2T | 43T | 21.6″ |
| 32T front · 29″ wheel | 25″ | 37.1T | 38T | 24.4″ |
| 34T front · 27.5″ wheel | 25″ | 37.4T | 38T | 24.6″ |
| 34T front · 27.5″ wheel | 30″ | 31.2T | 32T | 29.2″ |
| 34T front · 27.5″ wheel | 35″ | 26.7T | 27T | 34.6″ |
The arithmetic does not confirm compatibility. Before fitting a larger cog or different chainring, check derailleur maximum sprocket size, total capacity, chain length, freehub standard, frame clearance, chainline, and manufacturer instructions.
Compatibility source: SRAM's drivetrain service guidance explains why riders should verify cassette, derailleur, chainring, chain, and system compatibility before treating a mathematically suitable ratio as an installable setup.
Drivetrain comparison
Gear range is not the same as gear spacing
Overall range
range % = highest ratio ÷ lowest ratio × 100
A 500% range means the highest gear travels five times as far per crank turn as the lowest gear.
Spacing between gears
step % = next ratio ÷ current ratio − 1
Smaller steps make cadence changes gentler; larger steps cover a wide range with fewer sprockets.
Two drivetrains can have the same overall range but feel different because their steps, duplicated ratios, chainline, and shift sequence differ. Use the matrix to see the actual combinations rather than judging by the largest cog alone.
Questions answered
Bike gear calculator FAQ
How do you calculate a bicycle gear ratio?
Divide the teeth on the chainring attached to the crankset by the teeth on the selected rear sprocket in the cassette. A 50-tooth front ring with a 25-tooth cog has a 2.00 ratio, so a conventional chain-driven drivetrain turns the rear wheel twice for each crank revolution.
What are gear inches and gear development on a bicycle?
Gear inches combine the tooth ratio with wheel diameter. Gear development instead expresses the distance traveled per crank revolution, usually in meters, using wheel circumference. Both measures make gearing on bikes with different wheel sizes easier to compare.
Which combination is the easiest climbing gear?
The easiest gear normally combines the smallest available front chainring with the largest rear cog. It produces the lowest ratio and reduces distance traveled per pedal revolution, making pedaling easier on steep terrain. Confirm that the chain length and derailleur capacity support the combination.
Does wheel size change the bicycle gear ratio?
Wheel size does not change the tooth-count ratio, but it changes gear inches, rollout, and speed at a given cadence. A larger effective wheel travels farther per wheel revolution than a smaller one with the same sprockets.
Does a higher bike gear ratio always mean more bike speed?
A higher ratio increases theoretical bike speed at the same cadence because it travels farther per crank revolution. Actual speed still depends on terrain, wind, tire resistance, rider power, and whether the rider can sustain that pedaling rate efficiently.
Can this calculator be used for a fixed-gear or single-speed bike?
Yes. Enter one chainring and one rear sprocket. The calculator will report that combination's ratio, gear inches, rollout, and estimated speed at the selected cadence.
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Disclaimer
This bike gear calculator provides mathematical estimates for informational, comparison, and educational purposes. Actual rollout and speed depend on the fitted tire, inflation, load, wheel slip, drivetrain efficiency, measurement accuracy, and riding conditions.
Gear compatibility, chain capacity, chainline, derailleur limits, hub ratios, component wear, and safe installation are outside this calculator's scope. Confirm component combinations with the relevant manufacturer specifications or a qualified bicycle mechanic before buying parts or changing a drivetrain.
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